There are currently 3 full regions in PokeOne.Kanto, Johto, and Unova.
Each region has their own separate trainer level.This is to keep you from steam rolling through battles after completing a previous region, because you get to keep all your items and Pokemon when traveling between different regions.
A lot of people ask how do you get to Johto from Kanto and vice versa.You may remember entering a room (Pokemon League Reception) when you were on your way to defeating the Elite Four.You want to go back to that room.The North route takes you to Victory Road, the East route takes you to Kanto, the South route takes you to Johto, and the West route takes you to Mt. Silver (where you can battle Ash.)
How do you get to Unova?Currently, to get to Unova you have to be at least level 80 in Kanto OR Johto or you can get access by donating at least once (buying one of the loot box packages.)Once you meet one of these requirements you can head to Unova by boat at the Vermillion City Docks or Olivine City Docks (assuming you have defeated the Elite Four in that region.)Be careful though, once you go to Unova, you wont be able to get back to Kanto or Johto until you unlock Fly by defeating the 5th gym in Unova.